1. Field of the Invention
The present invention relates to a method and apparatus for stably melt-spinning thermoplastic polymer fibers having a superior uniformity and mechanical property by extruding a fiber-forming polymer into a decompressed atmosphere and then taking-up the polymer under normal atmospheric conditions.
2. Description of the Related Art
It is known in the prior art to melt-spin a fiber-forming polymer into a decompressed atmosphere. In principle, this type of spinning is advantageous in that a stable spinning operation can be carried out due to a decrease in the air resistance acting on a filament, and in that a yarn having an excellent mechanical property is obtainable due to an improvement of the skin-core structure of the filament.
Japanese Examined Patent Publication (Kokoku) No. 57-8206, discloses an apparatus in which a yarn is spun into a spinning tube having a decompressed atmosphere for minimizing any adverse influence from the ambient atmosphere on the yarn and for improving yarn uniformity. According to the prior art, in order to withdraw the yarn from the interior of the spinning tube, a yarn exit is provided in the vicinity of the bottom portion of the tube. The yarn exit comprises a plurality of sequentially arranged orifices. In this known apparatus, since filaments composing the yarn run through the spinning tube in a state in which they are separated from each other, an opening area of the yarn exit must be large enough to permit the yarn to smoothly pass therethrough. This causes a difficulty to arise of keeping the interior of the spinning tube under a high level vacuum. Further, due to the air flowing into the interior of the spinning tube through the opening of the orifice, the yarn in the spinning tube is caused to vibrate, and thus to come into contact with the orifice, which causes damage to the filaments composing the yarn from frictional wear and, in extreme cases, breakage of the filaments, thereby deteriorating the mechanical property of the resultant yarn. The above-mentioned vibration of the yarn further causes the filaments to become entangled with each other, which also disturbs the continuation of the stable spinning operation. Moreover, according to this prior art apparatus, the interior pressure of the spinning tube is at most 0.8 atm as disclosed in the example thereof.
Further to the above, in high speed spinning, since the extrusion rate of the polymer from the spinneret is necessarily increased for producing the identical yarn thickness compared to the conventional system and, coupled with this, the dwelling time of the yarn in the cooling zone is decreased due to the high speed take-up thereof, therefore, the cooling of the extruded molten polymer is insufficient before it is taken-up as a yarn. This drawback of insufficient cooling is promoted by the decompressed spinning tube because the mass of the gas in the spinning tube, which directly participates in the heat transfer from the yarn, is decreased. In addition, according to the apparatus disclosed in the abovesaid patent publication, the temperature of the air in the spinning tube is elevated as time passes because the heat transferred to the air from the yarn in stored therein and cooling of the yarn soon becomes difficult even if a means is provided for directly cooling the spinning tube. Further, monomer, oligomer, and catalyzer sublimated from the high temperature polymer flow tend to close the space within the spinning tube by separation and subsequent adherence to the inner wall thereof, which not only interferes with the smooth spinning operation but also degrades the effect of the heat exchange in the spinning tube.